The production of noxious nitrogen oxides (NOx) by internal combustion engines which pollute the atmosphere are undesirable and in many cases are controlled by regulations established by governmental entities. Furthermore, spark ignited engines exhibit abnormal combustion phenomena called “knock”, which occurs when combustion reactions in the unburned zone initiate rapid uncontrolled combustion prior to the arrival of the propagating flame front of a homogenous combustion process. One method for controlling knock includes increasing the flame propagation rate by, for example, improving the mixture homogeneity or by increasing the turbulence level induced by organized charge motion.
One method for limiting or controlling the combustion temperature of the engine and thus reduce NOx emissions has been to recirculate a portion of the exhaust gas back to the engine air intake to lower the oxygen content in the charge flow. This reduces the combustion temperature of the intake charge flow and in turn reduces the amount of NOx formation during combustion due to lower flame temperatures. In order to recirculate exhaust gas, an exhaust gas recirculation (EGR) line that connects the exhaust manifold to the intake air supply line is provided. One EGR method to increase the flame propagation rate is to have one or more cylinders dedicated to providing EGR flow to the engine intake. When the EGR line is connected with one or more dedicated cylinders, the engine acts as a positive displacement pump to drive the EGR flow, eliminating pumping losses in transporting exhaust to the intake system and allowing a wide range of engine out nitrous oxide emissions to be achieved. Also, since the exhaust from the dedicated cylinder does not escape the engine, it is possible to have alternative combustion processes with the dedicated cylinder(s), such as running the dedicated cylinder(s) in a rich combustion condition to generate a favorable species like hydrogen. In addition, a variable geometry turbocharger is not required to drive EGR flow, facilitating meeting of target air-fuel ratios.
However, these EGR arrangements come at the cost of a loss of control over the system, including a loss of control of the EGR fraction during low load and transient conditions. When nominal cylinders are dedicated to providing EGR, and standard fueling and controls are applied, the EGR fraction provided by the dedicated cylinders is limited to the simple ratio of the number of dedicated EGR cylinders to the total number of cylinders. For example, an engine with one cylinder dedicated to EGR and four cylinders total will operate at a 25% EGR fraction if all of the cylinders are operated in the same manner. However, during transient conditions the EGR fraction due to volume filling dynamics in the dedicated EGR flow path can vary significantly from the EGR fraction. In addition, in certain steady state conditions, the volumetric efficiency changes with load can cause deviations of the EGR fraction from its expected value. When the EGR fraction decreases from its expected value, knock conditions can be developed. When the EGR fraction increases from its expected value, cylinder misfire and combustion instability can result. Therefore, further technological developments are desirable in this area.